Error in terms.formula(object, data = data) : in Shiny App

Question

Im working in an linear regression shiny app and im having some troubles. The fisrt part is supposed to be an example with the swiss dataset. Here is the ui part:

  checkboxInput(inputId="example",
                label= "Let's see an example?",
                value=FALSE),
  conditionalPanel("input.example",
                   selectInput(inputId="swissy",
              label="Choose an response variable.",
              choices=names(swiss),
              selected=""),
  checkboxGroupInput(inputId="swissx",
                     label="Choose the explanatory variables.",
                     choices=names(swiss))
  ),

And the server part:

   observe({
    if(input$example){
      swissdata <- reactive({
      ys <- swiss[,input$swissy]
      xs <- model.matrix(~., swiss[,input$swissx])
      ms <- lm(ys~xs)
      return(ms)
      })
      output$table <- renderDataTable({swiss})
      output$model <- renderPrint({
        m0 <- swissdata()$ms
        print(summary(m0))
      })
      output$anova <- renderPrint({
        print(summary(anova(lm(swissdata()$ms))))
      })
      output$plots <- renderPlot({
        par(mfrow=c(2,2)); plot(lm(swissdata()$ms))
        ; layout(1)
      }, width=600, height=600)
    }
})

It should return, in tabsets, the summary of the linear model choosen by the user, the anova, etc. But the server is not recognizing the data, which is the dataset swiss. I dont know why this is happening,